1. Field of the Invention
The present invention relates to a production method of a rotating device and particularly to a production method of a rotating device, which improves the processing accuracy of a thrust dynamic pressure pattern and provides a rotating device with enhanced bearing stiffness, and to a rotating device produced by the method.
2. Description of the Related Art
There exists a rotating device such as a Hard Disk Drive (HDD) in which a fluid dynamic bearing unit, which allows for stable high-speed rotation, is mounted. An example of the structure of the fluid dynamic bearing unit is a fluid dynamic bearing unit provided with a thrust dynamic pressure generating site, with a space between a sleeve that forms a part of a fixed body and a thrust member that forms a part of a rotating body, the space being filled with lubricant. The thrust dynamic pressure generating site is provided with a thrust dynamic pressure pattern, obtained by forming an indentation of a predetermined shape on the surface of either the sleeve or the thrust member. The cross-sectional shape along the circumferential direction of the thrust dynamic pressure pattern is, for example, continuously folded-back, including indentations and protrusions therebetween. The fluid dynamic bearing allows for smooth and high-speed rotation by supporting the rotating body in a non-contact manner by the dynamic pressure generated in a part of the lubricant, by raking up the lubricant with the edges of the protrusions (for example, see patent document 1).
[Patent document 1] Japanese Laid-Open Publication No. 2008-275074
The stiffness of a fluid dynamic bearing is required to be further improved so that such a rotating device can be used for so-called mobile devices that are used in an environment subject to high vibration. Increasing the dynamic pressure of a thrust dynamic pressure generating site is effective for improving the stiffness. For this to be achieved, there are several options: narrowing down a gap between a sleeve and a thrust member; and forming the thrust dynamic pressure pattern to have a cross-sectional shape that allows for a lubricant to be more efficiently raked up. However, a thrust dynamic pressure pattern that is formed by plastic-working such as press-working has a deformed protrusion or may have protrusions of varying height.
When the height variation of the protrusions of the thrust dynamic pressure pattern is large, the area with the most height comes into contact with the opposite surface that the thrust dynamic pressure pattern faces when the gap between the opposite surface and the thrust dynamic pressure pattern is narrowed down, and the area with the most height is scraped due to contact with the opposite surface, which results in foreign particles. The existence of foreign particles in a bearing reduces the rotational accuracy. In the worst case scenario, a bearing malfunction, such as a burn-in, may be caused. Therefore, the gap between the thrust dynamic pressure pattern and the opposite surface cannot be greatly narrowed down. When the shape of the protrusion of the thrust dynamic pressure pattern is greatly deformed, a steep edge cannot be obtained, and the lubricant cannot be efficiently raked up. Therefore, the efficiency of dynamic pressure generation is low, and the stiffness of the bearing cannot be improved. In other words, a large deformation of the shape of the protrusion of the thrust dynamic pressure pattern and a large variation in the height of the protrusion prevents the stiffness of the bearing from being improved.